Expression syntax for `generated` fields

[dsebastien]

It would be valuable to improve generated to support temporal expressions beyond now.

Example:

fields:
  year:
    type: string
    generated: "format(now, 'YYYY')"
  quarter:
    type: string
    generated: "format(now, 'Q')"

The current now strategy only produces full datetime. Many use cases need just the year, month, quarter, or formatted date. Without this, tools resort to custom expression languages.

Spec section: Generated Field Strategies

[callumalpass]

Thanks for this suggestion! I like this idea--and I've felt the limitations created by its absence already. I'll work on adding this to the spec in the next update.